Currently marketed passenger vehicles are often equipped with a steer by wire steering device in which front wheels are mechanically decoupled from a steering wheel, and steered by a steering actuator in response to the operation of the steering wheel. In a steer by wire steering device, a steering unit having a steering actuator for turning the front wheels and a reaction force actuator for applying a steering reaction force to a steering wheel is provided. In order to allow the steering operation to be performed at the time of a failure, the steering unit and the steering wheel can be connected to each other via a clutch which is normally disengaged but can be engaged when required.
In such a steer by wire device, if the steering wheel is turned when the vehicle is stationary, the steering actuator consumes a significant amount of electric power. To avoid this problem, the steering actuator may be disconnected when the vehicle is stationary. To prevent the steering wheel from being readily turned while the vehicle is stationary, the steering wheel may be locked by supplying electric power to an electromagnetic clutch that engages the input end of the steering shaft to a fixed part. See JP2005-247171A, for instance.
JP2006-103580A discloses a steering control system in which the steering actuator is configured to act as a dynamic brake to keep the steering angle fixed and enable the vehicle to maintain a straight course by controlling a bridge circuit connected to the steering actuator in an appropriate manner. A dynamic brake is achieved by electrically short-circuiting two ends of the motor via a resistor.
JP2007-320537A discloses a steering assist motor that absorbs the impact of a vehicle crash by supplying electric current to the stator coil of the steering assist motor to produce an attractive force between the stator and the rotor.
When an electromagnetic clutch is used to lock the steering shaft when the vehicle is stationary, electric current is still required to be supplied to the electromagnetic clutch either to keep the clutch engaged or disengaged. More importantly, completely locking the steering shaft may cause discomfort to the vehicle operator.
The steering system disclosed in JP2006-103580A is a steering assist system, and is therefore not applicable to a steer by wire system.